Microfluidics generally refers to microfabricated devices, which are used for pumping, sampling, mixing, analyzing and dosing liquids. Prominent features thereof originate from the peculiar behavior that liquids exhibit at the micrometer length scale. Flow of liquids in microfluidics is typically laminar. Volumes well below one nanoliter can be reached by fabricating structures with lateral dimensions in the micrometer range. Reactions that are limited at large scales (by diffusion of reactants) can be accelerated. Finally, parallel streams of liquids can possibly be accurately and reproducibility controlled, allowing for chemical reactions and gradients to be made at liquid/liquid and liquid/solid interfaces. Microfluidics are accordingly used for various applications in life sciences.
Many microfluidic devices have user chip interfaces and closed flowpaths. Closed flowpaths facilitate the integration of functional elements (e.g., heaters, mixers, pumps, UV detector, valves, etc.) into one device while minimizing problems related to leaks and evaporation.
The analysis of liquid samples often requires a series of steps (e.g., filtration, dissolution of reagents, heating, washing, reading of signal, etc.). For portable diagnostic devices, this requires accurate flow control using various pumping and valve principles.
For many applications (diagnostics, etc.), reagents need to be integrated inside the microfluidic chips. Unfortunately, the dissolution and mixing of reagents inside microfluidics are often challenging and difficult to control and/or optimize. In microfluidics, laminar flow in a microchannel tends to dissolve reagents extremely fast and efficiently, which causes dissolved reagents to concentrate in a small volume of liquid. These reagents might therefore be too concentrated and/or present in an insufficiently large volume of liquid. Thus, a few mixing concepts have been introduced, mostly for mixing reagents along the width of microchannels, using e.g., active elements (valves, microstirrers, electrokinetic mixers, electroacoustic principles, recirculation of liquid and reagents in circular chambers, etc.). Such approaches, however, require external controllers and peripherals, interconnects to microfluidic chips (e.g., for pneumatic, electrical, and/or mechanical actuation) and, more generally, add complexity to the design, fabrication and packaging of microfluidic devices, which in turn raises costs of fabrication, makes microfluidic devices significantly more complicated to use, and the microfluidic device and peripherals bulkier and less portable.